temperatures which may be measured when an absorption glass of suitable density is employed. If still higher temperatures are desired a denser absorption glass must be used.
With the Wanner pyrometer the tip of the flame of the amyl-acetate lamp should burn level with the top of the flame gage. The setting on the normal point is tedious work as the flame flickers over the field. A screen of black paper placed around the lamp helps to reduce the flicker. The observations must be made in a closed room free from drafts. Any error in the setting of the normal current is carried over to the final temperature measurements so that it is exceedingly important to exercise all possible care in these preliminary adjustments. Examine the screen of the flame gage to assure that no smoke has deposited upon it. A slight film of smoke from the lamp may cause an error of 100° or more. The amyl acetate used in the lamp need not be of high purity. Any good grade of amyl acetate is satisfactory.
From experience with several hundred instruments in use in the technical industries it is evident that these pyrometers are subjected to great abuse. The instrument is made up of delicate optical parts and should not be allowed to become heated. Many of the parts are set in wax and the various optical surfaces are cemented by Canada balsam. The Wollaston prism and the nicol prism in the rotating eye piece are made of calcite. This material is very soft and, although it has the appearance of glass, may be scratched with the finger nail. In practically half the instruments examined these parts have been deeply cut by knives or pointed steel tools. All persons using this pyrometer should be cautioned not to touch any optical part except the lens in the eye cup which requires occasional cleaning. Do not change the setting
FIG. 19. — F. & F. pyrometer.
of any screw. The moving of a single screw may throw out the adjustment of the pyrometer and cause errors of 500°. If the position of any screw on the body of the instrument is altered, do not attempt to readjust the instrument but return it to the maker. Also do not take the instrument apart to find out what is wrong unless you are perfectly familiar with the optics of such a polarizing system, for it will gain nothing and the chances are that damage will be done. The replacement of the electric lamp will not alter the calibration of the pyrometer.
The Scimatco pyrometer, formerly sold by the Scientific Materials Co., is an improved form of the Wanner pyrometer. All but one of the screws, the tampering with which affects the calibration of the instrument, are enclosed in a metal sheath, and hence are not readily accessible. The instrument has both an angular scale and a scale graduated directly in degrees of temperature. With the Wanner or Scimatco pyrometer the observer can not see through the instrument the object sighted upon. This may cause inconvenience if it is desired to measure the temperature of a small crucible in a furnace.
Figure 19 illustrates the arrangement of optical parts in the F. and F. pyrometer made by the Scientific Materials Co. Light from the furnace is focused at the center of the silver strip cube C. This cube produces a circular field divided